Xeroderma pigmentosum (XP) is a hereditary disease in humans. It is characterized by extreme sensitivity to sunlight and to sun-induced skin disorders, such as actinic damage and cancer. Cultured cells from XP patients have defective DNA repair and are more sensitive to UV light than cells obtained from healthy individuals. The specific DNA repair defect in XP cells has not been characterized, but ample evidence suggests that it may be in the incision step of the nucleotide excision repair pathway. Cell-free extracts of XP cells, complementaton group A, have been shown to be incapable of excising UV-dimers from their endogenous DNA (presumably present as chromatin), but they can excise UV-dimers from exogenous purified DNA as well as can normal cell extracts. This suggests that excision of UV-dimers from DNA associated with chromosomal proteins, as opposed to purified DNA, may require additional ezymes or cofactors, and that they may be defective in XP cells. The objectives of this research are to: 1) examine the abilities of sonicated cell-free extracts from different XP complementation groups and from XP variant cells to excise thymine dimers from their endogenous chromatin; 2) perform in vitro complementation experiments with those XP complementation groups that are shown to be defective in excision repair; 3) investigate the dimer excising ability of normal and XP cell-free extracts prepared by methods other than sonication; and 4) use purified chromatin as a standard substrate for dimer excision studies with normal and XP cell-free extracts. These studies may lead to a better understanding of the molecular DNA repair defect in XP cells and provide some insight into the role that DNA repair plays in human carcinogenesis.